Spectral reflectivity of an object is an important optical characteristic of an object surface and is also a “fingerprint” or “root cause” of a color of the object surface. Therefore, color measurement is an indispensable main means for controlling color reproduction quality in the color reproduction-related industry.
Conventional color measurement methods of an object surface include three methods: a visual method, a photoelectric integration method, and a spectrophotometric method. The details are as follows: (1) In the visual method, a standard colorimetric observer performs visual discrimination on a product in a particular lighting condition, and makes a comparison with a Commission International de l'Eclairage (CIE) standard chromaticity diagram or other standard color cards to obtain a color parameter; (2) In the photoelectric integration method, a typical instrument is a colorimeter, including four parts: a light source, a detector, a data processor, and an output unit. The detector is generally a color sensor, such as three phototubes each having a correction filter set or a silicon photoelectric diode with a large area. The method cannot accurately measure tristimulus values or chromaticity coordinates of an object, cannot provide spectral reflectivity of the object, but can accurately measure a color difference between two objects. Therefore, the colorimeter is also referred to as a color difference meter, and cannot be used for accurate color reproduction; (3) In the spectrophotometric method, an integrating sphere spectrophotometer is usually used to make a to-be-measured object illuminated evenly; and grating splitting is used to obtain monochromatic light, and tristimulus values of a color are further calculated by measuring spectral reflectivity of reflected (scattered) light of an opaque object or spectral transmittance of a transparent object, so as to obtain various color parameters. The method is used to determine color parameters of a to-be-detected sample by detecting spectral components of the to-be-detected sample, and therefore has quite high precision. Therefore, the method is applicable to measurement of reflectivity of an object surface or transmittance of a transparent material during most color measurement.
The inventor found that the foregoing three conventional color measurement methods all have disadvantages. The disadvantages lie in that: The visual method is highly correlated with individual psychology and physiology of an observer, the measurement has strong subjectivity, low precision, and complex operations, and there is a relatively large difference between different observers; while the photoelectric integration method and the spectrophotometric method both are a contact measurement method, that is, a measuring aperture of an instrument needs to be tightly attached to a surface of a to-be-measured object, to prevent external light from entering the measurement instrument; and therefore, a primary characteristic of a surface of a fragile object is easily damaged, a fragile valuable high-temperature high-humidity object cannot be measured, and an article that possibly pollutes or damages the instrument, such as food, oil paint, and printing ink, also cannot be measured. A most criticized limitation of conventional color measurement methods is that: Only a target object with a specific size can be measured, and an object color of a tiny object, for example, a single pixel in an image, cannot be measured; and fine color reproduction for cultural relics, biomedicine, 3D printing, and the like cannot be implemented. All these are key technical problems that need to be resolved in current color detection methods.
Therefore, a color measurement method is needed to overcome the limitations of existing color measurement instruments, measure a color of a tiny object such as a pixel on an object surface in a non-contact manner, and for use in fields in which the conventional color measurement instruments cannot be applied, such as measurement of food and beverage, liquid, cosmetics, an object with a rough and uneven surface and an irregular shape, and the like.